


Spectral Font (one-shot)

by CathedralMidnight



Category: Undertale (Video Game)
Genre: Bondage, Daddy Kink, Doctor Roleplay, Finished, M/M, Magic Bondage, One-Shot, Pure Smut, Spanking, Teasing, Test run, Vaginal Sex, cameras recording, characters are not alternate version of each other, consent given, consent given previously, dirty talk/insults during sex, heat - Freeform, magic exchange, magic genitalia, proof of concept, sexy experiment, skeletal species, skeletons boning
Language: English
Status: Completed
Published: 2017-05-05
Updated: 2017-05-05
Packaged: 2018-10-28 09:32:11
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,279
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/10828524
Author URL: https://archiveofourown.org/users/CathedralMidnight/pseuds/CathedralMidnight
Summary: Red is in Heat and Sans decides to help him through it while also recording it. For science, of course!





	Spectral Font (one-shot)

**Author's Note:**

> Hello, all. This is a one-shot/proof of concept I wrote a few months ago to help me work out the whole skeletal species thing. Since I felt that the Red and Sans in this story have different personalities than they do in Soul and Bone, I decided this story would work better as a stand alone one-shot story than being part of the collection.
> 
> Other than a few mentions to other species, this story is pretty much just a straight forward heat scene. You don't have to read any of Soul and Bone to just enjoy the skeletons boning. :3
> 
> Enjoy!

He closed the door with a soft click. "How are you doing, test subject?"  
"Pl-please... I can't... take it... anymore...."  
"Calm down, shhh," he murmured. "I just need to observe you for a bit longer. Let's have a look." He walked over to a notepad on a bedside table, careful not to knock over the tripods holding the cameras. "Let's see... it is now... three-thirty A.M., and you have been in heat since eleven twenty P.M. of the previous day." He crawled into the bed in front of his "test subject".

His test subject sat up on his feet, legs spread wide in a squat by the restraints. His hands were bound thanks to the Barrier magic the smaller skeleton had cast on him. He couldn't have the test subject interfering with the natural process of his body after all.

"So, let's start from the top," the short skeleton gave a lazy grin. He leaned up on his knees to look into his test subject's sockets. "Ahh, your little ankh pinpoints have changed color. Can you tell me what shade of pink this is?"  
"Ho-hot pink," he breathed.  
"Ah, I see. Leave it to a Spectral Skeleton to know the exact shade of a color," he smiled. "And your Soul matches, how interesting," he noted on his notepad. "Now... shall we take a peek below?" He leaned down on the bed to take a closer look at the hot pink genitalia the skeleton had conjured. "My, dripping wet," he noted and ran a phalange over the dripping hole.   
"Ah, Sans! I---uh!"  
"Ah, ha, Test Subject Red," Sans smirked, phalange gracing over a swollen bud. "It's "Doctor" while the experiment is running, remember, test subject?"  
"I-I'm sorry, Do-doctor," Red breathed out, trying to lean more into the restraints that were forcing him into the squat. "Ple-please...."  
"Now, now, I still need to do more examinations," Sans spoke, sitting up. "Let's see.... Profuse sweating. That's fairly common. Mmm, open your mouth please and stick out your tongue for me? Ah... fangs are the same length. Tongue matches your pinpoints and Soul.... Are you in any pain?"  
"N-no, Doctor."  
"Very good," Sans replied, jotting down more notes, small smile on his skull. "So, Spectrals don't exude scents while in Heat like Fonts and Roses do. Your Color changes like Muse Skeletons. Interesting, very interesting." Sans took his time finishing up his note taking much to Red's disdain. He slid off the bed and placed the notepad back on the table. "Let me just check the cameras." He spoke, walking around the small refurbished shed. "How are you doing, test subject? You're not too cold, are you?"  
"I... I'm fine, Doctor," Red panted.   
Sans smirked. "Good. I had to put this all together in a rush, especially the restraints. How are those?"  
"F-fine, Doctor."  
"Don't worry, I'll undo the ones keeping you in the squat before we begin."  
"Uh, but my hands, I..." Red glanced away.   
"You know the rules. I can't have you touching yourself during the experiment."  
"But I want to touch you while we do it," Red murmured. "...Well, it's fine. I just have to get through the Receptive Phase first...."  
"And after that?" Sans glanced over at him.  
Red gave a toothy smirk, two silver replacement fangs glinting in the low light. "Oh, come now, dear "Doctor," isn't part of the fun of running an experiment the surprises?"  
Sans shifted his teeth. "Right. Let's start then. Cameras are all set up and running. Is the test subject ready?"  
"Ready, "Doctor,"" Red smirked.  
"Are you sure?" Sans asked, walking over and taking off his lab coat.  
Red sighed. "Yes, Sans, for the last time."  
"Okay, okay, I just have to make sure," the smaller skeleton replied, crawling into the bed. He reached around Red's wide pelvis and unfastened the restraints, allowing him to sit back on the bed.   
"You ready?" Sans asked again, taking off his white button up shirt.  
"Gods, yes! Will ya just fuck me already!?" Red cried.  
"So eager," Sans smirked, slipping out of his pants.  
Red stared down at the pulsing blue member in front of him. The ankhs in his sockets flashed and he ran a neon pink tongue over his fangs.  
"Calm down," Sans smirked, crawling in between Red's leg bones. "Here ya...ah... go...."  
"Ooooooh Gooooods Yessssss," Red droned, skull lulling back against the head board, sockets lidding, tongue drooling.  
Sans began to pump away. "You did a great job conjuring up this pussy. It's really hot and tight."  
"Uh...ah... yer cock feels good, daddy."  
"Oh, does my big bones have a daddy kink? That's... interesting coming from you."  
Red gave a curt laugh. "It's sick, right? I'm the last monster in tha Underground who should enjoy anything concernin' a dad, right?"  
"It's a dominance thing," Sans explains, still pumping. "Dads are in a place of authority, so what you really want is someone to have a little power over you." Sans smirked. "You're kinda a punk, so it's not really a surprise. So, yeah, I'll be your daddy tonight."  
"Man, fuck ya and yer psychoanalyzin' bullshit," Red growled, squirming under Sans' thrusting cock. "Just, fuck... keep fuckin' me, daddy, just like that!" he cried.  
"Sure, sure, just for you, you spoiled brat," Sans smiled, thrusting away. "Ah, your pussy's so tight.... How'd you...?"  
"Yers is a pretty good example," Red grinned.  
Sans blushed, glaring. "You would do something like that."  
"What, copy tha best pussy I've ever had? Fuck yeah, I'd do that." Red gave a knowing smile. "Now, hurry up and come inside me, daddy!"  
"Don't tell daddy what to do, kiddo, or you'll get the belt."  
"Ooooh, that actually sounds kinda hot."  
"Eh, maybe later," Sans spoke, taking a hold of Red's pelvis. "Right now, I gotta--ah, yeah...."  
"Ah, daddy! That's it, daddy! Faster, faster!" Red cried  
"Ah, man... okay, you... you want this inside, right?"  
"Yeah, yeah! Fill me up, daddy, please!" Red begged.  
Sans pumped faster, deeper, harder. "Almost... almost... allll... moooossst....fuuuuu...!"  
"OOOOOH, DADDY!" Red screamed as Sans magic flooded his pussy, mixed in with his magic, turning the hot pink to a more magenta shade. "Ah, fuck, yeah. Fuck me more, daddy."  
"You're so greedy," Sans breathed.  
"I'm _Receptive._ There's a difference," Red corrected, before smirking. "Now, get me on my hands and knees and fuck me like a two dollar whore."  
Sans grimaced. "Gods, calm down, Red."  
"I'm in _Heat_ , Sans, and I only have twenty-four hours before I go into ma next phase, so shut up, get me on my hands and knees and **FUCK. ME. LIKE. A. TWO. DOLLAR. WHORE."**  
Still grimacing, Sans pulled out, making Red shiver, and moved beside him to push him face first onto the mattress.  
He crawled behind Red as the bigger skeleton lifted his pelvis, exposing his dripping pussy. Sans took hold of Red's pelvis and slid inside.   
"Ohhhh yesssss," Red purred. "That's tha stuff." His tongue rolled out of his mouth onto the bed as Sans pumped away inside of him, thrusts long and deep. "Fuck, yeah, daddy.... Punish me for bein' a bad boy."  
Sans gave a nervous grin. "You're really getting into this, huh, kiddo. Well, how should I punish you?"  
"Spank me, daddy."  
Sans smirked, drew back his hand and popped Red one good time on his pelvis.  
 **"FUCK, YES!"** Red screamed. "FUCK, FUCK, YEAH, HARDER! PUNISH ME, DADDY! CALL ME A WHORE!"  
"You are a little whore," Sans grinned, slipping into a persona he didn't even know he had. "How many other monsters did you let fuck you before you got to me? I bet it was so many you can't even remember all the faces. You little whore, spreading your legs for whoever pops a dick out for you, huh?" He popped Red a few more times in pelvis, ripping out screams and moans from him. "Well, since you're mine now, I should make sure everyone knows it, right?"  
"Ah... make... make everyone know, daddy...," Red groaned.  
"Make everyone know what?" the smirk on Sans' face grew.  
"That I'm yers. I'm yers."  
"Say it louder." Another round of spanking.  
"I'M YERS! I'M YERS, I'M YERS! **I'M YERS!"**  
Right before he came, Sans rip his cock from Red's pussy. His magic shot out over Red's pelvis and lower spine, hot and thick. "There we go," he panted. "We might have to do that a few times to make it stick, though. You think you can handle that, kiddo?"  
"Yeah, yeah, daddy."  
"Good," Sans smirked, slipping his fingers inside Red, making him purr.  
So, twenty-four hours of this.  
Didn't seem as bad as Sans had first thought.

///

"Ah, Gods... okay," Sans breathed. "You good?"  
"Yeah, just gotta... let it sink in fer minute," Red answered. "If ya need ta do anythin', now's the time."  
"Right," Sans nodded, rolling out of bed onto the floor. "Ug... legs kinda...."  
"Sans, baby bones, I'm serious. Hurry up."  
"Okay, okay!" Sans scampered to his feet. "I'm just gonna switch out the tapes."  
"Fine, fine, just... shit...."  
Sans glanced back at the large skeleton lying face down on the bed, bones covered in blue magic from the many times Sans had "marked" him. He seemed to be shuddering a lot, hip bones jerking.  
"What's wrong?" Sans called while switching out tapes.  
"Nothin'. Just... goin' inta Phase Two."  
"What's Phase Two called?"  
Red laughed into a pillow, a low growl. "Tha Generosity Phase."  
"Generosity?" Sans questioned as he finished switching out tapes. He climbed back into bed just as Red rolled over.

The first thing Sans noticed was the massive cock in between Red's femurs. "...Wait, what color is that?" The shade was much darker than normal.  
"Wine," Red answered and Sans could see his Soul and Ankhs matched in color.  
"What the....?"  
"S'all part of the process, baby bones. Just means I've accepted all that magic---all that love--- ya gave me. So, now it's ma turn." Red rolled over onto the front his pelvis, sat up and then flexed his arm bones.

There was a sound of glass breaking as the Barrier magic Sans had used popped off Red's wrists.

Sans stared. "How did you---!?"  
"Well, I have Mastered ma magic," Red started, smirking. "I'm practically a boss monster in that regard, so breakin' out of a spell like that was easy--- just had ta overload it, like poppin' a balloon by fillin' it with too much air."  
"O-oh," Sans replied as Red loomed over him. "Are we... in Phase Two?"  
"Yep," Red smirked and pinned Sans to the bed by his wrists. "Here's how this is gonna go: Yer gonna conjure that tight pussy of yers I love so much and I'm gonna fuck ya and fill ya with so much love, that blue Soul of yers is gonna turn purple."  
"...Oh wow," Sans stared before his sockets narrowed. "Wait, "love"? You mean magic?"  
Red rolled the ankhs in his sockets. "Oh my gods, Sans, baby bones, yer a Font, ya oughta know what a metaphor is, yes, my precious baby bones, magic."  
"Well, I just---"  
"Oh my gods, stop talkin' and kiss me already!" Red leaned in, fangs pushing against Sans teeth, making the tiny skeleton shiver and moan.   
After a few minutes of this, Red pulled only centimeters away. "Pussy," he ordered.  
"Right," Sans nodded as his Soul lit up. The magic trailed down his spine to his pelvis, forming, as Red had said, the pussy he loved so much.   
Moments later, Red's phalanges slipped inside of Sans' tunnel making the skeleton moan and arc. Once Sans was dripping wet, Red slipped his enormous cock inside.

"Ah, yeah, there's that tight pussy I love fuckin'," he moaned.  
"Ah, uh... Red, you're... you're too big...," Sans squirmed.  
"Hold on, baby bones," Red whispered and pulled out. He went back in at a slower pace. "There we go, just let yer magic stretch around me. There ya go. Yer doin' great, baby bones."

Sans breathed, arms and legs wrapping around Red's sweaty bones as the larger skeleton thrust inside. Red had never been this big before and right now, Sans was grateful that Red preferred the front door a little more than the back, because, Gods, no matter how much Sans stretched his magic, Red just seemed to keep filling him.

"Red... Red, it... ah...."  
"I know, baby bones, it's a lot, but you'll be okay," Red whispered. "Soon, I'm gonna fill ya up. I'm gonna fill ya up with ever ounce of love I got for ya."  
"Red...," Sans sniffed. "Red, I... I love you."  
"I love ya, too, baby bones," Red whispered. "I'm gonna give ya some of it right now, okay? I'm gonna give ya... some of... it.. right... oh, fuck!"  
Sans cried out and arced as Red released a deluge of magic. Gods, his seed was so hot and thick and he could feel the magic coursing through his body, thundering through his bones, filling his Soul.

He burst into tears.

"I know, I know," Red whispered, kissing Sans' wet cheekbones. "It's a lot the first time, I know. But that's how I feel about ya, that's how happy ya make me. I wanted ta burst inta tears when ya agreed ta do this with me, but, ya know, appearances and all that."  
Sans giggled as Red kissed his cheeks. "You are such a big, bad bones."  
"Yeah, and yer a baby bones, so we balance each other out. I mean, look at ya little bones. Ya make me want ta eat ya up, little baby bones."  
"Hey, I'm older than you," Sans murmured against Red's fangs.  
"Still got baby bones, though," Red growled, smirking. "Certainly does make sex interestin'."  
"Oh, gods," Sans blushed.   
"I mean, I can just hold ya in my lap any way I want and do ya."  
"Come on, don't...."  
Red kissed Sans forehead. "Ain't my fault yer so short, with ya little baby bones. I'm gonna eat ya up one of these days." He humbled on Sans' vertebrae.  
"Ah, no...," Sans blushed.  
Red smirked. "Sans."  
"Ye-yeah?"  
"I love ya."  
Sans blushed again. "I love you, too."  
"I need to keep you fillin' ya up, though."  
"O-okay," Sans nodded. "I'm ready."

///

Sans murmured as his cellphone rang. "What? No... I told everyone I was running an experiment...." He reached over to the dresser for his phone. "Hello?"  
"Sans! Oh, gosh, are you okay?"  
"...Papy? Yeah, why wouldn't I be?"  
"The sign on the shed is still up, the one saying you were still running an experiment. I got worried because you said it'd be a two day thing, but I came by yesterday, and the sign was still up.   
"Well, yeah, I'm...," Sans sat up. "What day is it?"  
"The thirteenth."  
"Thirteenth? That can't be..." Sans looked at the calendar on the wall. _The ninth was when Red went into Heat. Receptive phase was all day and night on the tenth... The Generosity Phase should have been just the eleventh, but..."_ Sans blinked. _Did Red... have an extra day of Heat?_

"Sans?" Papyrus' voice brought him back.   
"Uh, sorry, Pap.... Yeah, it ran longer than planned. I'm sorry if I worried you. Let me just clean up here and I'll see you at Grillby's, okay?"  
"Well, all right. See you soon, brother!"  
"See ya," Sans hung up. "Red, big bones, wake up."  
"Hrm...," Red rolled over. "Hmmm.... oh...." He sat up, bones creaking. "Oooh, boy. Mornin', baby bones."  
"Red, I... I think you had an extra day of Heat," Sans stared up at the bigger skeleton.  
Red yawned. "Oh, yeah? Hn. Cool."  
Sans blinked. "Did you know about this?"  
"Yeah," Red answered. "S'rare, though."  
"But why didn't you tell me anyway?"  
" 'Cause ya wanted an average and I gave ya an average: two days."  
Sans looked down, brows furrowed. "Then why did we go an extra day?"  
"Hm, well," Red rubbed the back of his skull. "So, during ma Receptive Stage, the magic that ya gave me is responsible for giving me the boost I need to get through the Generosity Phase. But, that magic is given in an intimate way, so, it's the equivalent to love to us. So, ya gave me a lot of love while I was Receptive. Ya gave me so much, that I got an extra day to be Generous to ya."  
"....Oh," Sans blushed.  
"Yep," Red pulled Sans into his arms. "Just means ya love me a lot, baby bones. Hey, let's see how much I love ya," Red craned Sans' skull upward. "Ah, yeah," he smirked. "Those are some nice violet S's ya got in yer sockets."  
Sans sockets widen. "What?" He stumbled out of bed over to the mirror on the far side of the shed. "WHAT!?"  
Red laughed. "I told ya I'd fill ya you up so much love, your Soul would turn purple---well, violet, ta be more exact."  
"My---" Sans looked down at his ribs. "...Oh my Gods! It is purple!"  
"Violet," Red corrected. "That actual shade of purple is violet."  
"I---Red, what!?"  
"Don't worry, baby bones," Red waved off his concern. "S'temporary. Just, huh, be careful if ya do any magic, cause it might be a tad bit... stronger than normal."  
"Why did this happen!?" Sans cried.  
"Magic exchange," Red spoke. "Same reason my Soul turned from "red" to "wine" on the first day. I took in all yer magic, all yer love. Ya did the same fer me. Ta further demonstrate," Red popped out his Soul. "That is... watermelon pink. I'm so drained of magic, my Soul is a color that's not even a normal Soul Color. Kinda weird it does that when I'm in Heat, too, though...."  
Sans turned back from the mirror. "Are you... are you gonna be okay?"   
"Aye, yeah, Just need, like, a ton of food," Red stretched.  
"Okay...," Sans turned back to the mirror. "But what if someone ask about this?"  
"Tell them the experiment was a success."  
Sans turned back around. "What?"  
"This is the result ya want after spending Heat with a Spectral Skeleton. Ya _want_ yer Soul to be a different Color," Red explained. "So, if anyone ask, tell them ya ran an experiment and these were the desired results."  
"Oh...," Sans murmured. "Okay, then... if that's the case, then I hereby end this experiment and claim the procedure a success."  
"Good job, fam," Red clapped, making Sans S-pinpoints roll.   
He walked over and tapped Red's skull. "Get outta bed so we can clean up and get some food," he spoke, before stopping the tape in a nearby camera.   
"Red smirked. "Gonna jack off ta those later?"  
Sans jumped. "No! These are research tapes! For research!" he cried, blushing violet.  
Red's smirk grew. "Okay, sure, baby bones, whatever ya say."  
"Besides," Sans murmured. "Why would I do when I can just... you know...."  
Red blinked before smiling. "Aw, baby bones." He got out of bed and walked over to Sans, where he picked him up and squeezed. "Ooooh, I love you, too, baby bones."  
"Ugh, I'm like, two hundred years old! Stop calling me that!" Sans squirmed.  
"Yeah, I guess I should respect ma elders, but that's hard when ya so cute and tiny," he nuzzled against Sans' cheekbones.   
Sans sighed, purple blush on his cheekbones. "You are the only monster I would let do this to me."  
"Course I am, baby bones," Red smiled, kissing Sans' cheekbones. "Ya can punish me later for being such a brat, daddy. Make me a good boy," he whispered.  
Sans blushed more before smiling. "Hm.... Obedience training. That might make for a fun experiment."

**Author's Note:**

> I hope you enjoyed the sexy! I wanted there to be a nice contrast between the raw lust and pure love Red has during Heat. I think I achieved that well enough.
> 
> For those of you who are like, "this is great, but when TWH?" I'm pretty close to the written halfway point, so very soon! :3
> 
> Thank you for reading!


End file.
